In the semiconductor manufacturing industry, fabrication of integrated circuits on a semiconductor wafer involve a number of steps in which patterns are formed in a film of photosensitive resist, i.e. a photoresist, formed on the wafer. With the patterned formed, and void areas within the photoresist film, subsequent processing operations such as implantation of impurities, oxidation, etching and metallization may be performed. Once an integrated circuit is completely formed on a semiconductor wafer, the wafer is next assembled into a package. Solder bumps are typically formed on the topside of a wafer after a passivation coating has been applied to the integrated circuit formed on the opposite side of the same wafer. A favored method for forming solder bumps on a wafer's surface includes the use of a dry film photoresist, also referred to as a dry film resist (DFR) that is typically part of a composite layer including a carrier film and cover film. The DFR is a negative-type photoresist or photosensitive resin. After a solder bump pattern is formed in the DFR using exposure and developing methods, solder is electroplated into the void areas of the pattern. Before the solder can be reflowed, the DFR must be removed. The ability to completely strip the DFR depends on chemical characteristics but also physical effects especially when the solder bump array includes a fine pitch and a high aspect ratio is therefore present.
From a chemical reaction point of view, conventional methods for chemically stripping the DFR have low reaction rates due to the low collision frequency between the chemical stripper and the dry film photoresist molecules. Furthermore, from a thermodynamic point of view, the activation energy of photoresist molecules dissolving into the chemical stripper is intensified under static conditions, thus resulting in residues left on the wafers in view of the fact that an incomplete chemical reaction takes place in a limited processing time. Conventional processing suffers the risk of redepositing the dissolved impurities back onto the wafer in a viscous static chemical stripper solution due to the low solubility of the photoresist.
Conventional methods for attempting to strip DFR from wafers typically involve one or more chemical baths in which are placed cassettes or other carriers filled with a plurality of photoresist coated wafers, i.e., batch processing is used. A DFR pre-soak is carried out in a first tank and a second tank provides heating and agitation functions to strip clear any unremoved DFR and other residuals. The processing in the two tanks is then followed by further batch processing in a QDR (Quick Dump Rinse) chemical bath to clean the remaining chemicals and an IPA (isopropyl alcohol) dryer tank to dry the wafers.
The chemical stripping operations carried out in chemical baths have been found to be largely ineffective for removing DFR especially when fine-pitch bump solder arrays are used, for example arrays in which the bump to bump pitch is less than 175 microns. Other approaches for batch processing the wafers through multiple operations include the necessity for adding additional chemical baths because the chemical bath approach for stripping DFR particularly in fine pitch bump arrays, is generally insufficient.
The present invention addresses these shortcomings.